1. Field of the Invention
The present invention relates to a field effect transistor formed on a semiconductor thin film formed on an insulating substrate represented by SOI (silicon on insulator), polycrystalline silicon on a glass substrate, and SOS (silicon on sapphire), and to an integrated circuit thereof.
2. Description of the Related Art
Conventionally, in a MOS field effect transistor (hereinafter, abbreviated as a MOS transistor) formed on the SOI etc., if a silicon thin film portion referred to as a body where a channel is formed is in a floating state, at the time of increasing a drain voltage, a strong electric field generated between a drain and the body causes a current to flow therebetween, so that the current flows into a source from the body. Due to this inflow of current, the body and the source are subjected to a forward bias and a gate threshold voltage Vth of the MOS transistor is lowered. Further, this current is amplified through a parasitic bipolar transistor where the source is used as an emitter and the body is used as a base, and a current is further attained from the drain operating as a collector in the parasitic bipolar transistor. Through a positive feedback phenomenon like this, a drain current is abruptly increased at a certain drain voltage or higher, so that the MOS transistor using the body in a floating state is decreased in withstand voltage. In addition, even in a range of the drain voltage lower than that causing an abrupt increase in the current, there is caused an increase in an output conductance which adversely affects a voltage amplification factor of an analog circuit. A typical output current increase phenomenon is called a kink effect which is exhibited by the drain current being increased stepwise at 3 to 4 V in the voltage applied between the drain and the source.
For the purpose of improving the phenomenon, in order to fix the body at a constant potential, conventionally used are a T-type transistor structure as shown in a plan view of FIG. 1, an H-type transistor structure as shown in a plan view of FIG. 2, a source tie structure as shown in a plan view of FIG. 3, and an underlying or embedded body contact structure as shown in a sectional view of FIG. 4.
In the figures, reference numeral 111 denotes a drain region having a first conductivity type; 121, a source region having the first conductivity type; 131, a body contact region having a reverse conductivity type; and 400, a conductive gate region. Reference numerals 113, 123, 133, and 403 denote contact holes formed on the drain region, the source region, the body contact region, and the gate region, respectively. Through the contact holes, the respective regions are connected to metal thin film wirings 501, 502, 503, and 504, respectively. As shown in FIG. 4, below the gate region 400 between the drain region 111 and the source region 121, a gate insulating film 200 and a portion 100 corresponding to the body where the channel is formed are formed. In FIG. 4, reference numeral 10 denotes a supporting substrate; 102, an embedded or underlying body portion; 20, an insulating layer for allowing an insulation between the supporting substrate and a semiconductor thin film (consisting of the drain region 111, the source region 121, the body contact region 131, the portion 100, and the underlying body portion 102); 300, so-called field insulation films for isolating elements from each other; and 310, insulating layers for insulating the wiring and the semiconductor thin film from each other.
As shown in the T-type structure of FIG. 1 and the H-type structure of FIG. 2, the body portion is connected to the body contact region 131 through a portion below the gate region between the body contact region 131 and the source and drain regions. In these structures, the body contact region is arranged symmetrical to the source and the drain regions, which enables a so-called bi-directional circuit operation where functions of the source and the drain interchange each other. On the contrary, in the source tie structure of FIG. 3 and the underlying body contact structure of FIG. 4, the source region and the body contact region are connected, which does not allows exchanging the function between the source and drain and allows only a so-called single-polarity or one-directional circuit operation.
In both the T-type and H-type structures as described above, the electrical potential of the body is fixed through a body contact electrode 503 to realize a usable drain to source voltage (more than a few volts) by preventing so-called floating body effect. However, the body contact region is formed at an end portion in a gate width direction through the body below the gate. Also, in the source tie structure, the body contact region 131 is formed at both ends of the source in the gate width direction.
Therefore, if a gate width W of the transistor is increased, in the T-type transistor, the resistance between the body contact region 131 and the farthest portion to the body from the contact regions on the opposite side thereof becomes high, with the result that an effect due to the fixed body potential is weakened. Also in the H-type transistor and the source tie transistor, if the gate width W is increased, at a central portion of the body under the gate, the effect due to the fixed body potential is weakened.
The underlying body contact structure is a structure in which a contact portion 130 and the body 100 below the gate are continuously arranged through a portion below a source 120, so that if a source junction part reaches a deep portion of the film, the underlying body portion 102 between the body contact region and the body below the gate is increased in resistance, with the result that the effect due to the fixed body potential is weakened. In the future, since a technology for making the semiconductor thin film is advancing toward further thin, it is unavoidable that the resistance in the underlying body portion is increased.
Also in the above-mentioned T-type and H-type transistors, there has been a problem in terms of a circuit application. That is, the advantage that the bi-directional circuit operation is possible is applicable only in a range of a so-called reverse polarity with respect to the body contact potential. Therefore, once the potential of, for example, a p-type body is fixed, it is impossible for the source and the drain to operate securely at a negative potential with respect to the above potential (to be strict, at a negative potential exceeding the forward voltage of a pn junction). Thus the conventional T-type and H-type transistors also suffered from single polarity circuit operation.